(1) Field of the Invention
The present invention relates to a rotorcraft rotor, and also to a rotorcraft provided with such a rotor.
The invention thus lies in the technical field of rotorcraft rotors. More specifically, the invention lies in the field of abutments that are fitted to such rotors in order to limit the flapping movement of lift elements carried by the hub of such a rotor.
(2) Description of Related Art
A rotorcraft usually has at least one rotor for providing the rotorcraft with at least some of its lift and possibly also with propulsion.
A rotor comprises a hub that is set into rotation by a mast. The hub then carries at least two lift assemblies. Consequently, each lift assembly has a blade that is connected to the hub by means of a retention and mobility member. In particular, each blade may comprise a lift element that is fastened to a cuff, or indeed a lift element that has a cuff that is incorporated therewith.
For example, a retention and mobility member may include a hinge referred to as a “spherical abutment”. Each spherical abutment may have one strength member fastened to the hub and another strength member fastened to a cuff, regardless of whether the cuff is or is not incorporated in a lift element.
The blades are thus substantially plane elongate elements that are carried transversely by the hub. The blades are mounted on the hub by individual members for mounting the blades on the hub. The individual member includes the retention and mobility members for connecting the lift assemblies to the hub.
The hub may be a hub that has a plurality of plates, and in particular two plates secured to a mast. By way of example, one strength member of each retention and mobility member may be fastened to both of the plates by a respective pin.
In another embodiment, the hub may have a single plate that is fastened to the mast, with that plate having radial arms defining openings. Each retention and mobility member is optionally positioned in each of the openings, being secured to the plate via one of its strength members. A cuff then extends on either side of the plate going from the retention and mobility member towards a lift element.
The mounting members are fitted respectively to each of the blades at their root ends in order to allow them to be controlled by an operator. For example, mounting the blades in movable manner on the hub enables a rotorcraft pilot to act in flight to cause the pitch of the blades to vary collectively or cyclically in order to have an influence on the behavior of the rotorcraft with respect to its lift and/or to its propulsion.
The ability of the blades to move relative to the hub allows them to perform lead/lag movement, pitch movement, and upward and downward flapping movement. The notions of up and down should be considered relative to the direction in which the axis of rotation of the rotor extends. While the rotor is rotating at a nominal speed, the blades are spontaneously driven upwards under the effects of centrifugal force and of lift. When the rotor is stopped, the blades are subjected only to the force of gravity and they thus tend naturally to droop downwards because of their weight.
The rotor is thus commonly fitted with an abutment mechanism that limits the amount of movement the blades can perform in flapping under the effect of forces external to the rotorcraft.
In one embodiment, the abutment mechanism may comprise, for each blade, a lower abutment member and an upper abutment member that forms obstacles to the individual flapping movements of the blades respectively downwards and upwards. The abutment mechanisms include a lower abutment track and an upper abutment track that are arranged on a retention and mobility member. For example, one of the strength members of a spherical abutment may carry an abutment skid having an abutment track.
In the event of unexpected flapping movement of a blade, an abutment track comes into contact with an abutment member, thereby limiting the flapping movement of that blade.
A particular function of the abutment mechanism is to limit the flapping movement of the blades when starting the rotor, and more particularly so when in the presence of wind. The abutment mechanism may also limit the flapping movement of the blades during an operation of folding the blades. In particular when in the presence of spherical abutments hinging respective blades of the hub, each abutment mechanism tends to protect the corresponding spherical abutment while on the ground by preventing a blade in the absence of centrifugal force from performing a flapping movement that might give rise to delamination of the elements making up the spherical abutment.
While the rotor is rotating, the upper abutment members must not oppose upward movement of the blades. The abutment mechanism may then provide means for retracting the upper abutment members while the rotor is in service.
The upper abutment members are then movably mounted on the hub to move between two positions. A first position is an engagement position for the upper abutment members relative to the corresponding abutment tracks in order to act while the rotor is stationary to limit the ability of the blades to move in flapping between the lower abutment members and the upper abutment members associated therewith. A second position is a disengagement position for the upper abutment members, thus making it possible in flight for the blades to move freely in upward flapping.
To summarize, an upper abutment mechanism usually includes one abutment member per lift assembly, an abutment track carried by a lift assembly, e.g. by its retention and mobility member and refraction means capable of positioning an abutment member so that it faces an abutment track, when necessary.
Furthermore, the abutment mechanism includes a low abutment ring that is common to all of the blades. This low abutment ring is movable in a groove.
In flight, only one blade at a time can move downwards, and it pushes the ring in an opposite direction. The ring therefore does not impede flapping of the lift element.
In contrast, when on the ground, with the rotor rotating slowly or stationary, the lift assemblies come simultaneously into contact with the ring, thereby mutually preventing them from moving.
An abutment mechanism for a blade thus comprises a low abutment ring that is common to all of the lift elements and one retractable high abutment per lift element. Such a ring is sometimes referred to as a “reciprocal ring”.
Those two abutment systems are independent and tend to increase the number of mechanical parts needed in a space that is restricted by its very nature.
Furthermore, on a heavy aircraft, the low abutment ring may present weight that is not negligible, and in flight it may generate unbalance that needs to be taken into consideration.
Document U.S. Pat. No. 2,719,593 describes a rotorcraft provided with a plurality of blades hinged to a rotor head, in particular via a “drag” hinge that allows the blade to perform lead/lag movement, a flapping hinge, referred to as a “flapping link”, that allows the blade to perform flapping movement, and a pitch hinge that allows the blade to perform pitch movement. The flapping hinge is interposed between the pitch hinge and the drag hinge.
For each blade, the aircraft has a flapping retention mechanism for holding the blade in a given position. That mechanism comprises a hook connected by vertical arms to the flapping hinge and constrained to move in flapping with the blade. The hook is secured to a weight in order to respond to centrifugal force by tilting about a horizontal axis of rotation that is hinged to the vertical arms.
The hook comprises a U-shaped jaw defining a groove, the U-shape being obtained with an arcuate bottom wall and two symmetrical vertical walls. The jaw is then engaged around a peg of the drag hinge when the rotor is stationary in order to prevent upward flapping of the blade, and is disengaged from the peg when the rotor is put into rotation.
The peg is then secured to the blade during its lead/lag movement, the hook being secured to the blade during its lead/lag movement and its flapping movement.
That document U.S. Pat. No. 2,719,593 thus does not provide teaching about a lift assembly connected to a hub by a spherical abutment constituting a three-axis hinge.
Such a spherical abutment presents the particular feature of its component elements becoming delaminated in the event of flapping movements taking place in the absence of a high level of centrifugal force.
Also known are the following documents: FR 2 636 914, U.S. Pat. No. 4,368,006, FR 2 434 079, FR 2 297 166, U.S. Pat. Nos. 3,533,713, 2,928,478, 2,614,640, 2,481,747, 2,151,215, 4,549,852, FR 2 551 723, and FR 2 523 071.
Document FR 2 636 914 describes a mechanism having a low reciprocal abutment ring that is common to all of the lift elements, together with one retractable high abutment per lift element. A high abutment comprises a lever having two arms, a first arm constituting an abutment finger and a second arm carrying a flyweight.
Document FR 2 434 079 describes a mechanism having a low abutment reciprocal ring common to all of the lift elements and one high abutment per lift element, which high abutment comprises a lever with a bend.
Document U.S. Pat. No. 4,368,006 suggests using a lever that has a rounded surface.
Document FR 2 297 166 describes a lift element secured to a fork that is fastened via a spherical elastomer bearing to a hub and via a short shaft to an elastomer centering bearing.
A first stop element is free to move in pivoting about the short shaft and includes a local projection having a first flat abutment surface. A second stop element is supported by the hub and includes a local projection having a second flat abutment surface suitable for preventing downward flapping of the lift element in flight by interfering with the first flat abutment surface.
A static droop stop is provided to limit flapping of the lift element while the rotor is stopped or rotating at a low speed of rotation. That static droop stop is provided with a contact element that is connected to a spring and that is suitable for coming up against the first flat abutment surface.
Document U.S. Pat. No. 2,614,640 describes a lift element connected to a hub via a drag hinge and via a flapping hinge and a pitch hinge like Document U.S. Pat. No. 2,719,593.
An abutment mechanism comprises a flap that is movable in pivoting relative to the drag hinge. That flap is for limiting downward flapping of the lift element by interference with a surface of the flapping hinge.
That flap is movable in pivoting by a lever having a flyweight.
Document U.S. Pat. No. 4,549,852 describes a lever that is movable in pivoting relative to a hub. That lever is suitable for moving a piston in order to block a high flapping track or a low flapping track of an elastomer bearing.
Document FR 2 551 723 describes an abutment mechanism having distinct low and high abutment devices for each lift element.
Each low abutment device comprises a movable member having a roller presenting a convex abutment surface and a flyweight arranged at the end of an arm. The movable member is mounted to pivot about a pin for retaining a lift assembly to a hub.
Each high abutment device includes a nose and a lower flyweight that is returned by a spring.
Document FR 2 523 071 describes abutment means supported by a hub. The abutment means are movable in pivoting relative to the hub under the effect of centrifugal force and in opposition to the action of resilient means. The abutment means are provided with two members that tilt about axes that are substantially parallel.
Document U.S. Pat. No. 3,533,713 describes a rotor having a hub that is movable relative to a mast, the rotor being of the “gimbal” type. The rotor is also provided with a mechanism for blocking the hub in a position relative to the mast. That document is thus remote from the technical field of blade abutments.
Likewise, Documents U.S. Pat. Nos. 2,928,478, 2,481,747, and 2,151,215 describe devices for blocking the angle of inclination of a hub that is capable of pivoting relative to a rotor mast.
Also known are Documents US 2011/243734 and FR 2 760 425.